Tower of Heaven looks and sounds as if it could be a Gameboy title: flat gray (or actually) green-scale graphics and the kind of Midi music track you'd expect from such a title. What it actually is, however, is a little Gamemaker platformer notable for one thing: a highly polished, and frustratingly difficult set of level challenges. (At least frustratingly difficult for me, but as I've noticed before, I suck at this kind of game.)
In other words, there's nothing particular innovative about the setting or the mechanics, but the level design is both excellent and extremely tight.
The story, such as it is, is that you are attempting to scale the tower of some enormously powerful being who smites you if you fail to complete each level in the time allotted. He also occasionally hands down a "Law" that increases the difficulty of solving a particular level -- e.g., "Thou shalt not move left." Naturally, the level is then designed to make getting through it a challenge with that particular rule in effect.
In short, Tower of Heaven is worth playing both for the fact that it's an enjoyable platformer in its own right, and also as an example of excellence in level design.